add a button to select an image

[jywarren]

This would be great in the demo: https://jywarren.github.io/fisheyegl/example/

along with the current version which triggers when you drag in an image.

Code links

The code for that is here:

https://github.com/jywarren/fisheyegl/blob/master/example/main.js#L104-L142

And here's an example of updating based on a file select button:

https://github.com/publiclab/image-sequencer/blob/master/examples/index.html#L196-L205

They're not too dissimilar; we need the handler to respond to both button-based and drag-based file selection. Any help appreciated!

Note that it's not identical -- the current code uses:

    var files = e.dataTransfer.files;

Whereas the example i linked to uses:

          var file = e.target.files[0];

Both use a FileReader() and reader.readAsDataURL(f);